I lost it in class today when the students were teasing a stutterer

I lost it in class today when the students were teasing a stutterer I teach English in northern China and teaching in a foreign country with very little English affords me a lot of stuttering cheats. I can use word replacement with no odd looks, I can speak very slowly or in a rhythm and it's considered polite and if I get suck I can just stop and again no one even thinks twice. Today I had the students, seventh grade, come to the front and individually do an exercise in front of the class and then this girl, came up and at first I thought she was just thinking about what you say, not every student as is adept as others, until I heard as total vocal stop. That "first syllable stuck" syndrome and the class starts laughing. I at first try to regain control and stop her from speaking so she can take her time but they keep saying things in Chinese that I know it's an insult. I flipped out, got an angry face for the first time in this class and slapped the main asshole in the back of his head. I shouldn't of done that but they got the idea. I let her finish, with my angry face to the class, but only made her do a half of the spoken sentence (I had eggs for breakfast) and said it with her. I moved on like nothing happened after that but man, I just lost it and I guess it brought back bad memories. I'm the future I'll have to handle her a little different then others but it makes me think, should I exclude her from public speaking assignments or press her more or treat her the same? Makes me think of the dilemma my teachers had when I downright refused to speak or read in front of the class I just wanted to share.